Solid-state imaging apparatus are well known as image sensors used in digital still cameras and digital video cameras. A solid-state imaging apparatus (called a CCD sensor or a CMOS sensor) includes a substrate, e.g., a silicon semiconductor substrate and an array of pixels containing a photodiode and reads signal charge corresponding to photoelectrons generated in the photodiode of each pixel by a CCD or CMOS drive circuit.
The known solid state imaging apparatus includes not only the photodiode but a signal charge storage part, a charge transfer part, a signal readout part, a wire connecting them, and so on formed on the semiconductor substrate. Therefore, as the pixel pitch becomes smaller, the area proportion of the circuit region in unit pixel becomes larger, which of necessity leads to the problem of reduction in the effective area of the photodiode, i.e., reduction of pixel aperture ratio. Reduction of aperture ratio leads to reduction of sensitivity.
To compensate for the reduction of aperture ratio, it would be essential to condense incident light through, for example, a microlens formed on each pixel thereby to obtain a substantially increased aperture ratio. However, this solution unavoidably incurs an increase in cost due to involvement of an extra production step and complexity of optical design, and yet produces only a limited effect on sensitivity.
To overcome the problem, a stacked or tandem solid state imaging apparatus has been proposed, in which a photoelectric layer is superposed on a semiconductor substrate having circuitry and wiring formed to increase the pixel aperture ratio. A representative structure of such a configuration includes a semiconductor substrate and a large number of photoelectric devices arranged two-dimensionally in parallel to the substrate, the photoelectric devices each including a lower electrode formed on the substrate, a photoelectric layer stacked on the lower electrode, and an upper electrode stacked on the photoelectric layer. With a bias voltage applied between the lower and upper electrodes, charges photogenerated in the photoelectric layer are swept toward the upper and lower electrodes. The signals corresponding to the negative or positive charges collected at either electrode are read out by a CCD or CMOS circuit embedded in the semiconductor substrate. The photoelectric layer is a layer that absorbs a specific wavelength or wavelengths of incident light to generate charges (electrons and holes) in response to the quantity of light absorbed.
A photoelectric apparatus having a photoelectric layer containing an organic material excellent in color reproducibility has been studied for practical use. Examples of conventional sensors having a photoelectric layer containing an organic material include a full color image sensor having a thin organic semiconductor film between conductive electrodes as disclosed in JP 2002-502120A (corresponding to U.S. Pat. No. 6,300,612 B1) and a sensor having a light sensing portion containing an organic material between electrodes as disclosed in JP 2005-311315A.